By the year 1994, the particulate emission standards set by the Environmental Protection Agency (EPA) will require all urban buses and heavy duty trucks to emit less than 0.1 gm/hp-hr of particulate matter. Particulates are defined by EPA as any matter in the exhaust of an internal combustion engine, other than condensed water, which is capable of being collected by a standard filter after dilution with ambient air at a temperature of 125.degree. F. Included in this definition are, agglomerated carbon particles, absorbed hydrocarbons, including known carcinogens, and sulfates.
These particulates are very small in size, with a mass median diameter of 0.5-1 micro meters, and are of very low bulk density. During the life of the typical vehicle, approximately 20 cubic feet of particulate matter which must be trapped will be emitted per 100,000 miles of engine operation. This amounts to approximately 100 lbs. of particulate matter or more depending upon the type of vehicle. Obviously this particulate matter cannot be stored within the vehicle because one pound of particulate occupies a volume of approximately 350 cubic inches. Therefore, there is a need for a filtration system which will both efficiently and reliably remove these particulates from the exhaust emission of these vehicles
One such solution to the above emissions problem is disclosed in U.S. Pat. No. 4,449,362 issued to Frankenberg et al. In the disclosed system, during normal driving conditions the exhaust gas from an internal combustion engine flows through an outer passage and continues through a filter positioned at the end of the system, where a portion of the particulate matter within the exhaust is trapped and the remainder is emitted to the atmosphere. When the system senses that a sufficient amount of particulates have been collected, a portion of the exhaust gas stream is directed to flow through an inner flow passage and through an electrical heater and a catalyst bed. The catalyst bed is provided with an aspirating device which mixes fuel with the exhaust flow to raise the temperature of the catalyst bed to approximately 1200.degree. F. This temperature is sufficient to cause the carbon particulates retained in the filter to begin burning. Upon completion of this burning cycle the exhaust is again routed through the outer passage. It should be noted, that the excess exhaust flow during the burning cycle is vented directly to the atmosphere. By positioning the catalyst bed between the filter to be regenerated and the fuel supply, the catalyst bed is directly subjected to the aspirated fuel as well as extremely high temperatures. This can result in inhibiting formations of sulfates as well as the possible burn out of the catalyst which will lead to expensive repair or require replacement of the entire system.
In U.S. Pat. No. 4,485,621 issued to Wong et al a similar system for reducing particulate emissions from internal combustion engines is disclosed. Again, a catalyst is positioned upstream of a particulate trap and directly subjected to aspirated fuel. This fuel is combined with a portion of the exhaust and expended through the catalyst and raised to a temperature of 600.degree. C. This heated mixture is then directed through the particulate trap in order to oxidize the particulate matter retained therein. Again, by subjecting the catalyst to the aspirated fuel as well as the high temperatures, unwanted sulfates may form thereon resulting as well as possible burn out of the catalyst.
A further attempt in capturing emitted particulates within a particulate trap and system for regenerating the particulate trap is disclosed in U.S. Pat. No. 4,677,823 issued to Hardy. This system includes a particulate trap positioned within an exhaust stream, downstream of a diesel fuel burner used for the purpose of regenerating the particulate trap. During normal operation engine exhaust is routed through the particulate trap to a muffler located downstream thereof, and then expended to the atmosphere. Once a sufficient pressure build up is sensed by the control system, the regeneration cycle will commense. At this time the exhaust gas is directed through the by-pass conduit, through the muffler and expelled to the atmosphere. Diesel fuel is aspirated within the diesel fuel burner to form a fuel-air mixture which is ignited by a spark plug in response to the condition sensed by the control system. The burning mixture is maintained at a temperature between 1200.degree. F. and 1400.degree. F. so as to properly oxidize the particles retained in the trap. This mixture, as well as the particles dislodge from the trap and not sufficiently oxidized, are then also expelled to the atmosphere In doing so, these particles along with the exhaust gas expelled during the regeneration cycle are emitted directly into the atmosphere Without any further treatment. These untreated emissions may result in detectable particulates in excess of the new EPA standard which will be unsatisfactory for use in specified vehicles by the year 1994.
As is clear from the above, there is a pressing need for an exhaust particulate trap and regeneration system which will both significantly and reliably reduce the amount of emitted particulate from diesel engine exhaust so as to comply with the future standards set by the EPA.